In the context of this paper the 'user' includes all those involved in the management of a geothermal resource. The objective of reservoir modelling and simulation, in my view, should be to supply these 'users' with appropriate and reliable data so that they can make sound decisions, whether these be in financial or engineering fields. The scale of models would thus extend from a single well, which may require a workover to remove mineral deposition costing, say, $30,000, to a f u l l geothermal field development, costing more than $100 million.

Geothermal development work can involve high risks. With the aid of modelling it should be In the context of this paper the 'user' includes all those involved in the management of a geothermal resource. The objective of reservoir modelling and simulation, in my view, should be to supply these 'users' with appropriate and reliable data so that they can make sound decisions, whether these be in financial or engineering fields. The scale of models would thus extend from a single well, which may require a workover to remove mineral deposition costing, say, $30,000, to a full geothermal field development, costing more than $100 million.

Geothermal development work can involve high risks. With the aid of modelling it should be possible to quantify risks, and, if these are considered excessive, to identify what data should be obtained to reduce them. Schemes relying on a single well are particularly hazardous. In 1974 an alfalfa drying plant was built at Broadlands next to well BR7. This well had previously been flow tested for three years, showing very reliable discharge characteristics. But when opened to supply the drying plant, the well did not perform satisfactorily and a 350 m pipeline was built to supply steam from another nearby productive well. All geothermal developments involve drilling and this, also, is a risky business. In New Zealand, over the last six years, the time to drill a 1200- 1400 m investigation well shows variations of 240% about the average (using the same rig and equipment). The time for workovers is even more variable, due to unforeseen downhole conditions affecting progress - an overrun of 100% would not be unexpected.

What information does the user want? Basic data will be the power-life and likely discharge characteristics of a field given certain operating conditions. For example, if we have a liquid dominated field, the design engineers will require information about changing enthalpy, gas and total flow for the system as a whole and for each individual well. There is no point in fitting a well with equipment to handle 100 t/h of separated steam if, after three years' production, it will only produce 50 t/h. Those providing finance for a scheme will require an assurance of its viability. How many years can the field sustain the projected withdrawal rate? For a well repair operation, those providing funds will want to know the chances of success and how much steam the well might produce if the operation succeeds. These factors must be balanced against the estimated cost of the operation, which itself relies on the problem having been properly identified in the first place. In a field which has been operating for some years, the question arises: What do we do now? Do we increase production, or do nothing, or maybe manage the field by operating only selected wells. Whatever the choice, predictions of future field and well performance are required to allow the field operator to evaluate the economics of alternative schemes.

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